Command Explanations

--enable-threads: This
switch forces the package to build with thread support.

make
install-private-headers: This command is used to
install the Tcl library interface
headers used by other packages if they link to the Tcl library.

ln -v -sf tclsh8.4
/usr/bin/tclsh: This command is used to create a
compatibility symbolic link to the tclsh8.4 file as many packages
expect a file named tclsh.

sed -i -e ...
tclConfig.sh: The Tcl package expects that its source tree is
preserved so that packages depending on it for their compilation
can utilize it. This sed removes the references to the
build directory and replaces them with saner system-wide locations.